Apparatus for detecting a failure in automatic wire extension

ABSTRACT

Detecting a failure in automatic wire extension in an electric discharge machine performing wire extension through a small-diameter machining initial hole by using a subnozzle attached to a main nozzle. During wire extension through a small-diameter initial hole, a voltage is applied to the input terminal of a first short-circuit detecting circuit (161) connected to a main nozzle (120) for normal wire extension, the first detecting circuit is connected to a control unit (170) through a logic circuit (166-169), and a second short-circuit detecting circuit (162) for detecting a short circuit between a wire (40) and a workpiece (50) is disconnected from the control unit. A failure in the wire extension is detected after the wire reaches the subnozzle (150), while permitting the wire to touch the machining initial hole (51). When the passing of the wire is blocked so that the wire touches the main nozzle, occurrence of a wire extension failure is detected in response to a signal from the first short-circuit detecting circuit whose the input terminal is grounded via the main nozzle and the wire.

TECHNICAL FIELD

The present invention relates to an apparatus for detecting a failure inautomatic wire extension of a wire cut electric discharge machine.

BACKGROUND ART

Wire cut electric discharge machines generally comprise upper and lowerwire guides which are respectively disposed above and under a worktablefor fixing a workpiece and are individually provided with a wire passagefor guiding a wire electrode (hereinafter merely referred to as wire).These machines are arranged to produce an electric discharge between thewire and the workpiece, while running the wire previously insertedthrough the wire passages and a machining initial hole formed throughthe workpiece, to thereby carry out electric discharge machining, andare further arranged to inject a machining fluid from upper and lowernozzles, respectively provided at the upper and lower wire guides,toward an electric discharge machining region. When a disconnection ofthe wire occurs during the electric discharge machining or when the wireis intentionally cut upon completion of the machining for each productduring the manufacture of a plurality of products from one workpiece,the electric discharge machines operate to extend the thus accidentallyor intentionally disconnected wire between the two wire guides, bycausing the wire to be fed through the upper wire guide whilerestraining the wire by the machining fluid jetted from a wire extensionnozzle attached to the upper nozzle, thereby causing the wire to beinserted into the initial hole of the workpiece and then into the wirepassage of the lower wire guide.

In case that such automatic wire extension is carried out successfully,the wire does not touch the workpiece during the wire extension until itreaches the lower wire guide. On the other hand, when the wire insertionto the initial hole or to the lower guide is blocked and thus the wireextension ends in failure for the reason, for instance, that a burrexists in the inner peripheral surface of the initial hole or sludge isaccumulated in the wire passage of the lower wire guide, the wire isflexed and touches the workpiece. In this respect, conventionally, theautomatic wire extension is performed with a predetermined voltageapplied to the wire, and this wire voltage is monitored, so as todetermine an occurrence of a failure of the wire extension when the wiretouches the workpiece which is grounded, namely, a short circuit occursbetween the two, and accordingly the wire voltage is dropped to theground potential. When such a wire extension failure occurs, a similarwire extension operation is repeatedly carried out, and if the wireextension still fails despite a predetermined number of times of wireextension, the wire is extended through another initial hole.

In case that the wire extension is performed through an initial holewith a relatively large diameter suitable for manufacture of ordinaryproducts, the wire extension can be achieved at a practicallysatisfactory rate. On the other hand, when the wire extension iseffected through a small-diameter initial hole suitable for manufactureof products requiring extremely precise machining, such as lead framesof integrated circuits, the chance of succeeding in the wire extensionis lowered. In view of this, the applicants hereof proposed a novel wireextension method (Japanese Patent Application No. 63-252893), whereinwhen carrying out a normal wire extension through an initial hole havinga normal diameter, the wire electrode is fed while the machining fluidis injected to the workpiece from a main nozzle positioned at apredetermined distance from the workpiece, and when carrying out aspecific wire extension through a small-diameter initial hole, asubnozzle having a nozzle hole diameter smaller than the small diameterof the initial hole is attached to the main nozzle, and the wireelectrode is fed with the subnozzle positioned closer to the workpieceand with the injection of the machining fluid interrupted. According tothe wire extension method of this type, the wire is more likely to touchthe workpiece when inserted through the machining initial hole, andtherefore, a wire extension failure cannot be accurately detected by theaforementioned type of wire extension failure detecting method in whichthe presence/absence of short circuit between the wire and the workpieceis monitored. As a result, the retry function of the electric dischargemachine to retry the wire extension upon occurrence of a wire extensionfailure cannot be effectively used.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an apparatus fordetecting a failure in automatic wire extension, which is installed in awire cut electric discharge machine for automatically carrying out wireextension through a small-diameter machining initial hole by using asubnozzle attached to a main nozzle.

According to the present invention, an apparatus for detecting a failurein automatic wire extension, installed in a wire cut electric dischargemachine, comprises: a short-circuit detecting circuit for generating apredetermined detection output when a short circuit occurs between awire electrode and a main nozzle during execution of wire extensionthrough a machining initial hole having a small diameter by use of asubnozzle mounted to the main nozzle and electrically insulatedtherefrom; disabling means for disabling the short-circuit detectingcircuit to render the predetermined detection output therefromineffective after the start of the wire extension until the wireelectrode reaches a predetermined delivery position; and discriminationmeans for determining that the wire extension ended in failure when thepredetermined detection output is generated by the short-circuitdetecting circuit after the short-circuit detecting circuit is releasedfrom a disabled state.

Preferably, the wire cut electric discharge machine is arranged to carryout a second wire extension through a machining initial hole having anormal diameter by using the main nozzle; and the discrimination meansdetermines whether the first-mentioned wire extension or the second wireextension is being executed. The automatic wire extension failuredetecting apparatus further includes a second short-circuit detectingcircuit for generating a second predetermined detection output when ashort circuit occurs between the wire electrode and a workpiece duringexecution of the second wire extension, and the discrimination meansdetermines that the wire extension ended in failure when the secondpredetermined detection output is generated.

As described above, according to the present invention, the unsuccessfulwire extension is determined when the predetermined detection outputrepresenting a short circuit between the wire electrode and the mainnozzle is generated by the short-circuit detecting circuit after thewire electrode reaches the predetermined delivery position after thestart of the wire extension through a small-diameter machining initialhole by the use of the subnozzle attached to the main nozzle andelectrically insulated therefrom. Therefore, the determination of wireextension failure is not adversely affected even when the wire electrodetouches the workpiece, and a wire extension failure can be accuratelydetected even at the time of wire extension through a small-diametermachining initial hole. Consequently, the retry function of the electricdischarge machine to retry the wire extension, which is based on thedetermination of wire extension failure, can be effectively used, thusenabling a continuous operation of the electric discharge machine.Preferably, a failure in the wire extension through a machining initialhole having a normal diameter can also be detected, and accordingly, awire extension failure detecting apparatus suited to generalapplications can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary schematic front view of a principal part of awire cut electric discharge machine to which an apparatus for detectinga failure in automatic wire extension is mounted;

FIG. 2 is a longitudinal sectional view of a principal part of an upperwire guide of the electric discharge machine in FIG. 1, with a subnozzlemounted to the upper wire guide;

FIG. 3 is a fragmentary plan view of a portion of an arm assembly of anautomatic wire extension unit shown in FIG. 1;

FIG. 4 is a diagram showing an apparatus for detecting a failure inautomatic wire extension according to a first embodiment of the presentinvention; and

FIG. 5 is a diagram showing an apparatus for detecting a failure inautomatic wire extension according to a second embodiment of the presentinvention.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to FIGS. 1 through 4, a wire cut electric discharge machine towhich an apparatus for detecting a failure in automatic wire extensionaccording to a first embodiment of the present invention is mounted,comprises a Z axis unit 10 mounted to an upper column (not shown) in amanner vertically movable relative thereto, and a UV axis unit 20mounted to the Z axis unit 10 so as to be vertically movable in unisontherewith and horizontally movable relative thereto. An upper wire guide30 is mounted to the unit 20 for movement in unison therewith.

The upper wire guide 30 is, as a whole, formed into a hollow cylinder,and accommodates therein a pair of hold rollers 31 for holding a wireelectrode (hereinafter referred to as wire) therebetween and feedingsame downward during a wire extension operation, mentioned later. Thehold rollers 31 are arranged to be movable toward and away from eachother, andare coupled together through a one-way clutch (not shown)which is engaged when the rollers rotate in wire feed directions. Asshown in FIG. 2, the upper wire guide 30 is provided at a lower portionthereof with an annularholder 32 into which an upper nozzle 33 and a dieguide 34 having a wire passage (not shown) bored therethrough arefitted, these elements 32-34 being disposed in alignment with oneanother. Supplied to the upper wire guide 30 through a pipe (not shown)is a machining fluid, which is then injected toward a workpiece 50 fixedon a worktable (not shown) through the wire passage of the die guide 34and the upper nozzle 33.

The electric discharge machine further comprises a lower wire guide 70secured to a lower column 60 under the worktable. The lower wire guide70 has a lower nozzle 71 at an inner end thereof and accommodatestherein a three-point support guide (not shown), which is arranged inalignment withthe upper nozzle 33 and the die guide 34 accommodated inthe nozzle 33. At the downstream side of the lower wire guide 70 isdisposed a belt type wire conveying device 80 composed of a pair ofbelts, which are movable toward and away from each other and are adaptedto travel while being urged against each other with the wire 40therebetween during automatic wire extension, to thereby convey thewire.

The electric discharge machine is further provided with an automaticwire extension unit 90 having a housing 91 which is secured to the UVaxis unit20 and to which a motor and a first piston-cylinder assembly(neither is shown) are mounted. A rod 92, which is rotatable in unisonwith the rotaryshaft of the motor and axially movable relative thereto,has a lower end towhich one end portion of an arm assembly 100 issecured, so that the arm assembly 100 is swung by the motor and movedvertically by the first piston-cylinder assembly. As shown in FIG. 3, awire cutting unit 110 comprising a stationary cutter 111 and a movablecutter 112 is provided the outer side of the other end portion of thearm assembly 100 in the width direction of the assembly. A wireextension nozzle 120 comprising a stationary nozzle portion 121 and amovable nozzle portion 122 is providedat the inner side of same in theassembly width direction.

More specifically, the stationary cutter 111 is secured to a framemember 102 which is detachably fixed to one end of the arm assembly body101 by bolts, and the movable cutter 112 is secured to the distal end ofa pistonrod of a second piston-cylinder assembly (not shown) secured tothe main body 101, such that, as the piston reciprocates, the movablecutter 112 ismoved toward and away from the stationary cutter 111together with the piston rod. The stationary nozzle portion 121 isdetachably fixed to the frame member 102 by bolts, and the movablenozzle portion 122 is detachably fixed by bolts to the distal end of apiston rod 130 of a thirdpiston-cylinder assembly secured to theassembly body 101, so that, as the piston reciprocates, the movablenozzle portion 122 is moved toward and away from the stationary nozzleportion 121 in unison with the piston rod 130. The cutters 111, 112 andthe nozzle portions 121, 122 are so arrangedthat when they are set apartfrom their counterparts, a slit 140 is definedbetween the opposed facesof these elements to allow the wire 40 to be passed therethrough.Further, the nozzle portions 121, 122 are formed at their opposedsurfaces thereof with semicylindrical holes, inverted semi-conicalholes, small-diameter semicylindrical holes, and semicylindrical holesin the mentioned order from top to bottom, so as to define a cylindricalhole 123 into which an annular end 33a of the upper nozzle 33 is closelyfitted, an inverted-cone shaped hole 124 communicating with a nozzlehole 33b of the upper nozzle 33, a nozzle hole125 communicating with thehole 124, and a recess 126 into which a subnozzle 150 mentioned later isfitted, when the movable nozzle portion 122 is brought in contact withthe stationary nozzle portion 121.

The subnozzle 150 is electrically insulated from the main nozzle 120 andismade of an insulating material such as ceramic or the like. Thesubnozzle 150 is composed of two separate halves 150a and 150b,symmetrical with each other respect to a plane passing the axis of thesubnozzle and each consisting of a semicylindrical main portion 151a,151b and a semicylindrical extension 152a, 152b formed integrally andcoaxially with the main portion. The subnozzle halves 150a, 150b aredetachably fixed to the nozzle halves 121, 122 of the main nozzle 120,respectively, by appropriate means (not shown), so that, when thesubnozzle halves 150a, 150b are joined together, the joined mainportions 151a, 151b is fitted into the recess 126 of the main nozzle120, and one subnozzle half 150b ismovable in unison with the movablenozzle portion 122. The subnozzle halves150a, 150b are each formed atopposed faces with an inverted semi-conical hole, opening in the uppersurface of the main portion 151a , 151b, and a small-diametersemicylindrical hole, communicating at one end with the invertedsemi-conical hole and opening at the other end in the lower surface ofthe extension 152a, 152b. When the subnozzle 150 is fitted intothe mainnozzle 120, with the subnozzle halves 150a, 150b joined together, aninverted cone-like hole 153 and a small-diameter cylindrical subnozzlehole 154 are defined in alignment with the axis of the subnozzle, whichiscoaxial with the nozzle hole 33b of the upper nozzle 33. The diameterof the hole 153 is 1.5 mm, for example, at the upper surface of thesubnozzle, and the diameter of the same hole 153 at the extension sideandthe diameter of the hole 154 are 0.3 mm, for example.

The electric discharge machine further includes a numerical control unit170 constituting a part of a wire extension failure detecting apparatus160 described later. The control unit 170 is connected to variousoperating parts of the electric discharge machine, such as respectivepower sources (e.g., motor, piston-cylinder assembly, etc.) of themovableportions of the Z axis unit 10, UV axis unit 20, hold rollers 31,wiretable, movable sections of the three-point support guide, wireconveying device 80 and automatic wire extension unit 90, and isconnectedto an electric discharge power supply, a machining fluid supplysystem, anda sensor system.

With reference to FIG. 4, the apparatus for detecting a failure inautomatic wire extension, which forms an essential part of the presentinvention, will be now described.

The wire extension failure detecting apparatus 160 is operable in afirst detection mode for detecting a wire extension failure whencarrying out a wire extension through a small-diameter machining initialhole 51 by usingthe main nozzle 120 and the subnozzle 150 incombination, and a second detection mode for detecting a wire extensionfailure when carrying out a wire extension through a machining initialhole 51 with a normal diameter by using the main nozzle 120 alone. Tothis end, the detecting apparatus 160 is provided with first and secondshort-circuit detecting circuits 161and 162 of conventionally knowntype, including comparators and the like, and the numerical control unit170 which has the function of discriminating between the detection modesand determining the success/failure of the wire extension, in additionto the conventionally known function of controlling the variousoperating parts of the electric discharge machine.

The input terminal of the first short-circuit detecting circuit 161 isconnected to a DC voltage source +V of a predetermined voltage, e.g., 10V, through a first relay contact 163, and also connected to the mainnozzle 120 which is electrically conductive. The input terminal of thesecond short-circuit detecting circuit 162 is connected to the samevoltage source +V and also to the wire 40 through a contactor 164. Theconnection node between the second short-circuit detecting circuit 162andthe voltage source +V is grounded through a second relay contact 165.Further, the output terminal of the first short-circuit detectingcircuit 161 is connected to one input terminal of a first NAND circuit166 whose another input terminal and both input terminals of a secondNAND circuit 167 are connected to a computer (not shown) accommodated inthe numerical control unit 170 through an output circuit (not shown)accommodated in thethis unit. The output terminal of the secondshort-circuit detecting circuit 162 is connected to one input terminalof a third NAND circuit 168, the other input terminal of which isconnected to the output terminalof the second NAND circuit 167. Theoutput terminal of the third NAND circuit 168 is connected to one inputterminal of a fourth NAND circuit 169. Moreover, the other inputterminal of the fourth NAND circuit 169 is connected to the outputterminal of the first NAND circuit 166, and the output terminal of thecircuit 169 is connected to the aforesaid computer through an inputcircuit (not shown) accommodated in the numerical controlunit 170.

Reference numeral 171 denotes a manual operation means for the selectionofa detection mode, which comprises, for example, a manual data inputdevice ordinarily provided in the numerical control unit 170 andconnected to thecomputer through the input circuit. The computer isarranged to store flag information representing the selected detectionmode in a predetermined register when a detection mode is selected by anoperator by operating themanual data input device 171. Further, thecomputer is so arranged as to apply a detection mode select controloutput, through the output circuit, to driving coils of relays (notshown) respectively including the first and second relay contacts 163,165, and to one input terminal of the firstNAND circuit 166, when itreads, from an electric discharge machining program (not shown), apredetermined command to start a process for detecting a failure in wireextension. The select control output is also applied to one inputterminal of the third NAND circuit 168 after it is inverted by thesecond NAND circuit 167. The workpiece 50 is grounded.

Operation of the wire cut electric discharge machine having the aboveconstruction will now be described.

During an electric discharge machining operation, an electric dischargeis produced between the wire 40 and the workpiece 50 while the wire 40is caused to travel by the wire feeding device, not shown, with the armassembly 100 of the automatic wire extension unit 90 retracted sideways,and the machining fluid is injected from the upper and lower nozzles 33and 71 toward the electric discharge machining region.

In the case of manufacturing a plurality of products from a singleworkpiece 50, the wire 40 is cut every time the electric dischargemachining is completed for one product, and then a wire extension iscarried out in a mode according to the diameter of the machining initialhole 51 (FIG. 1) into which the wire 40 is to be inserted next. First, anormal wire extension through a machining initial hole having a normaldiameter, e.g., 2 mm or more, which is executed for producing anordinary product not requiring extremely precise machining or the like,will be described. In this case, the operator selects the seconddetection mode byoperating the manual data input device 171.

When the electric discharge machining for one product is finished, thevarious operating parts of the electric discharge machine aresequentiallyoperated as described below, in accordance with variouscontrol outputs from the numerical control unit 170.

First, the operation of the machining fluid supply system and wireconveying device is stopped, thereby interrupting the supply of themachining fluid and the feeding of the wire. Subsequently, the Z axisunit10 is raised to move the upper wire guide 30 upward, and a pair ofhold rollers 31 are driven to approach each other so as to hold the wire40 therebetween. The arm assembly 100, which is in a retracted position,is lowered by the first piston-cylinder assembly via the rod 92 of theautomatic wire extension unit 90, and then swung by the motor until thecutters 111, 112 come to a position just under the upper nozzle 33,whereby the wire 40 is introduced between the cutters 111 and 112through the slit 140. Next, the movable cutter 112 is driven by thesecond piston-cylinder assembly, to cut the wire 40. After the armassembly 100 is swung back toward the retracted position, the downstreamside portion of the wire 40 is held between a pair of belts of the wireconveying device 80, and then, the same device 80 and a wire feed roller(not shown)are driven to discard the wire in a wire recovery container(not shown).

Subsequently, the worktable is moved in a horizontal plane to a positionwhere the center of the machining initial hole 51, which is boredthrough the workpiece 50 and into which the wire 40 is to be inserted,is aligned with the axes of the upper and lower wire guides 30 and 70.The arm assembly 100 is again swung toward the upper wire guide, tobring the mainnozzle 120 to a position just under the upper nozzle 33.Next, the third piston-cylinder assembly is driven to bring the movablenozzle portion 122into contact with the stationary nozzle portion 121,and with the wire 40 held in the nozzle hole 125 defined by these twonozzle portions, the arm assembly 100 is raised such that thecylindrical hole 123 in the upper surface of the main nozzle receivesthe annular end 33a of the upper nozzle 33. Simultaneously, a movablepiece (not shown) of the three-point support guide is moved away from astationary piece of same (not shown), thus permitting the insertion ofthe wire 40. The Z axis unit 10 is then lowered until the distancebetween the extreme end face of the main nozzle120 and the upper surfaceof the workpiece 50 becomes equal to a predetermined value (11 to 13 mm)suited to a normal automatic wire extension.

At this stage, the computer of the numerical control unit 170 reads acommand to detect a wire extension failure, and applies an L-leveldetection mode select control output to the first and second NANDcircuits166, 167 and the two relays, in accordance with the flaginformation representing the detection mode set by the aforementioneddetection mode-setting operation of the operator. As a result, the firstNAND circuit 166 is disabled, and the third and fourth NAND circuits 168and 169 are enabled. Further, the first and second relay contacts 163,165 areopened, whereby a voltage of 10 V is applied from the voltagesource +V to the wire 40 and the input terminal of the secondshort-circuit detecting circuit 162 via the contactor 164, while theapplication of the voltage tothe main nozzle 120 is interrupted.

Under the above conditions, while the machining fluid is injected fromthe upper nozzle 33, the hold rollers 31 are rotated to feed the wire 40toward the machining initial hole 51 and the lower wire guide 70.Usually,the wire 40, restrained by the jet of the machining fluid,passes through the machining initial hole 51 and then through the wirepassage of the three-point support guide and between the stationarypiece and movable piece of same, to the wire feed roller via the wireconveying device 80, thus completing a wire extension process. Sinceduring a normal wire extension the wire 40 does not touch the workpiece50, a voltage of 10 V is continuously applied to the input terminal ofthe second short-circuit detecting circuit 162 and accordingly anH-level signal is continuously supplied to the third NAND circuit 168from the circuit 162. Therefore, anH-level signal representingnonoccurrence of a short circuit between the wire 40 and the workpiece50 (i.e., wire extension failure) is continuously supplied to thenumerical control unit 170 from the fourth NAND circuit 169.

Subsequently, after interrupting the supply of the machining fluid andstopping the rotation of the hold roller pair 31, the upper wire guide30 is raised together with the Z axis unit 10. At this time, the one-wayclutch provided between the two hold rollers 31 is disengaged and thusthehold rollers 31 run idle. The arm assembly 100 is then lowered andthe wireextension nozzle 120 is detached from the upper nozzle 33. Next,with the movable nozzle portion 122 and the stationary nozzle portion121 set apartfrom each other to permit passage of the wire 40therebetween, the arm assembly 100 is swung back and then is raised upto the retracted position. Simultaneously with this, the movable pieceof the three-point support guide is driven toward the stationary piecethereof to slidably hold the wire 40 therebetween, and the hold rollers31 and the belts of the wire conveying device 80 are driven in adirection such that they are moved away from their counterparts, therebyreleasing the wire 40 from therollers and the belts. Finally, the upperwire guide 30 is lowered to a predetermined height for preparation of anelectric discharge machining operation.

When a failure occurs in wire extension, a short circuit is causedbetween the wire and the workpiece 50, and thus the wire 40 is groundedvia the workpiece 50, causing a drop of the voltage at the inputterminal of the short-circuit detecting circuit 162 to 0 V. In thiscase, an L-level signal representing the occurrence of a wire extensionfailure is suppliedto the numerical control unit 170 from the fourthNAND circuit 169. Upon receiving this failure detection signal, thecomputer of the numerical control unit repeats the aforementionedoperation procedure to retry the wire extension.

Next, operation of the electric discharge machine will be describedreferring to the case of a specific wire extension through a machininginitial hole having a small diameter, e.g., 0.5 to 2 mm, which isexecutedfor producing specific products requiring extremely precisemachining, etc.In this case, the operator previously selects the firstdetection mode by operating the manual data input device 171, and setsthe setting value forthe distance between the distal end face of thesubnozzle 150 and the uppersurface of the workpiece 50 to 0.1 to 0.2 mm,which value is referred to bythe computer of the numerical control unit170 when the Z axis unit 10 is moved for height control.

To carry out the wire extension, the related parts of the electricdischarge machine are operated in accordance with basically the sameprocedure as that in the case of the normal wire extension alreadydescribed. However, in this case, prior to the execution of the wireextension, the subnozzle 150 is previously attached to the bottom of themain nozzle 120, and the machining fluid is not supplied for the wireextension.

Thereafter, the computer of the numerical control unit 170 reads thecommand to start the wire extension failure detection, and applies anH-level detection mode select control output to the first and secondNAND circuits 166 and 167 and the two relays, in accordance with theflag information representing the detection mode set by the aforesaidmode setting operation of the operator. As a result, the second NANDcircuit 167 is disabled, the first and fourth NAND circuits 166 and 169are enabled, and the first and second relay contacts 163, 165 areclosed, whereby the application of the voltage to the wire 40 and theinput terminal of the second short-circuit detecting circuit 162 fromthe voltage source +V is interrupted, while a voltage of 10 V is appliedto the main nozzle 120 from the voltage source +V.

Under these conditions, the hold rollers 31 are rotated to feed the wireof0.2 mm in diameter, for example, toward the machining initial hole 51having a small diameter (e.g., 0.5 to 2 mm) through the main nozzle 120fitted onto the upper nozzle 33 and through the subnozzle 150 having anozzle hole diameter of, for example, 0.3 mm, thereby starting asubstantial wire extension.

When the substantial wire extension is started, the computer startsdetecting a wire extension failure in the first detection mode accordingto the flag information representing the detection mode and inaccordance with the wire extension failure detection start command.First, the computer starts a timer (e.g., a software timer using aprogram), in whicha predetermined time has been set. The predeterminedtime is set at a valueslightly longer than a time period which isnormally required for the wire 40 to reach a predetermined wire position(in this embodiment, the inlet of the subnozzle hole 154 of thesubnozzle 150) from the time the substantial wire extension is startedand which is previously calculated based on the distance between theposition of the leading end of the wire at the time of starting thesubstantial wire extension and the predetermined wire position and thewire feed speed.

When the wire is fed toward the small-diameter initial hole 51 throughthe main nozzle 120 and the subnozzle 150, since the distance betweenthe nozzle 150 and the workpiece 50 is as small as 0.1 to 0.2 mm,usually the wire 40 can be smoothly inserted into the initial hole 51without strikingagainst the upper surface of the workpiece, even whenthe leading end of the wire is deflected due to the presence ofliability to coil up. As the wire 40 is further fed toward the lowerwire guide 70 thereafter, it is passed through the wire passage of thelower nozzle 71, the wire passage of the three-point support guideaccommodated in the lower wire guide 70, and between the movable andstationary pieces of the same guide, without any difficulty despite theliability of the wire to coil up, due to the wire guiding effect of thesmall-diameter machining initial hole 51. Then,the above-describedpost-processing is executed for preparation of an electric dischargemachining operation.

During the normal wire extension described above, an occurrence of awire extension failure is determined by the wire extension failuredetecting apparatus 160 in the following manner. Before the lapse of thepreset timeperiod, which is set in the timer and is slightly longer thanthe time normally required for the wire 40 to reach the inlet of thenozzle hole 154 of the subnozzle 150 from an instant at which thefeeding of the wire 40 from the nozzle hole 125 of the main nozzle 120is initiated, the computer ignores the detection output from the fourthNAND circuit 169 andmakes no determination of wire extension failurebased on the level of the same detection output. Accordingly, even whenthe wire 40 fed from the main nozzle 120 touches the inner peripheralsurface of the nozzle hole 125 of the main nozzle so that a voltage of 0V appears at the input terminal of the first short-circuit detectingcircuit 161 which is connected to the ground potential through the mainnozzle 120, the wire 40, the contactor 164, and the relay contact 165,and the resultant L-level output representing an unsuccessful wireextension is applied to the computer from the fourth NAND circuit 169,such an output representinga wire extension failure is ignored.

Thereafter, the determination of wire extension failure based on thepresence/absence of contact between the wire 40 and the main nozzle 120isstarted. To be noted, the wire 40 is slidably supported by thesubnozzle 150 made of an electrically insulating material and iselectrically insulated from the main nozzle 120 connected to the inputterminal of the first short-circuit detecting circuit 161. Accordingly,even when the wire40 is inserted into the machining initial hole 51 andtouches its inner peripheral surface, a voltage of 10 V still appears atthe input terminal of the first short-circuit detecting circuit 161, andtherefore, an erroneous detection of the occurrence of wire extensionfailure never occurs. During the normal wire extension, the wire 40 isinserted into thelower wire guide 70 without being flexed, and duringthis process, the output of the fourth NAND circuit 169 is maintained atthe H-level and hence the computer determines that the wire extension isbeing executed normally.

On the other hand, if the wire 40 is flexed when the wire is fed fromthe main nozzle 120 to the subnozzle 159 for the reason that the nozzlehole 154 of the subnozzle 150 is clogged, for instance, so that the wiremay remain touching the main nozzle 120 even after the lapse of thepredetermined time period set in the timer, the input terminal of thefirst short-circuit detecting circuit 161 is connected to the groundpotential through the main nozzle 120, the wire 40, the contactor 164and the second relay contact 165, as well as through the main nozzle120, the wire 40 and the workpiece 50. As a result, a voltage of 0 Vappears at theinput terminal of the first short-circuit detectingcircuit 161 and the resultant L-level signal is applied from the outputterminal thereof to the input terminal of the first NAND circuit 166.Accordingly, an L-level signal indicative of unsuccessful wire extensionis supplied from the fourth NAND circuit 169 to the computer, which thendetermines that a wireextension failure has occurred. Further, if anabnormal condition exists invarious wire insertion sections, forexample, a burr exists in the machining initial hole 51 or sludge isstored in the wire passage of the lower nozzle 71, the passing of thewire 40 is blocked. In this case, the wire 40 is flexed and finallytouches the main nozzle 120, and the occurrence of such wire extensionfailure is detected in the above-described manner.

When the wire 40 is disconnected by accident during an electricdischarge machining operation, automatic wire extension is executed asin the case of intentional cutting of the wire 40. In this case, upondetecting a wiredisconnection in a conventional manner, the downstreamside portion of the disconnected wire 40 is discarded, and then, the armassembly 100 is movedto the position for wire extension, shown in FIG.1, following the aforementioned procedure. The worktable is then drivensuch that the center of the initial hole 51 associated with machiningduring which the disconnection of the wire occurred, is brought intoalignment with the axes of the upper nozzle 33 and of a correspondingone or both of the mainnozzle 120 and the subnozzle 150. Subsequently,the wire extension is carried out, accompanied by the determination ofpresence/absence of unsuccessful wire extension effected by the sameprocedure as that mentioned above. These operations on this occasion areapparent from the foregoing description, and hence explanation thereofwill be omitted.

When switching is made from the specific wire extension mode using thesmall-diameter initial hole to the aforesaid normal wire extension modeusing the normal-diameter initial hole, the subnozzle 150 is detachedfromthe main nozzle 120 in advance.

With reference to FIG. 5, an apparatus for detecting a failure inautomaticwire extension according to a second embodiment of theinvention will be now described.

As compared with the detecting apparatus 160 of FIG. 4 including thefirst and second short-circuit detecting circuits 161, 162 and the logiccircuits provided between these detecting circuits and the NC unit 170,a detecting apparatus 160' according to this embodiment is different inthatit uses a single short-circuit detecting circuit 161' to attain asimplified arrangement.

Specifically, the input terminal of the short-circuit detecting circuit161' is connected to a DC power supply +V and a movable contact 163'a ofafirst relay. The movable contact 163'a is arranged to be selectivelyconnected to a first stationary contact 163'b of the first relayconnectedto the main nozzle 120, or a second stationary contact 163'cconnected to acontactor 164 and a stationary contact 165'b of a secondrelay. The movablecontact of the second relay is grounded.

Like the detecting apparatus 160 of FIG. 4, the detecting apparatus 160'shown in FIG. 5 is operated in either of the first detection mode fordetecting an unsuccessful wire extension during a wire extensionoperationusing the main nozzle 120 and the subnozzle 150 in combination,and the second detection mode for detecting an unsuccessful wireextension during wire extension using only the main nozzle 120. In thefirst detection mode, in accordance with a detection mode select commandfrom the NC unit 170, the movable contact 163'a of the first relay isconnected to the first stationary contact 163'b, and the movable contact165'a of the second relay is connected to the stationary contact 165'bto thereby connect the contactor 164 to the ground potential. In thesecond detectionmode, the movable contact 163'a of the first relay isconnected to the second stationary contact 163'c, and the movablecontact 165'a of the second relay is disconnected from the stationarycontact 165'b. The detecting apparatus 160' is operated similarly to theaforementioned detecting apparatus 160 in other respects, and therefore,explanations thereof are omitted.

What is claimed is:
 1. An apparatus for detecting a failure in automaticwire extension, installed in a wire cut electric discharge machine,comprising:a short-circuit detecting circuit for generating apredetermined detection output when a short circuit occurs between awire electrode and a main nozzle during execution of wire extensionthrough a machining initial hole having a small diameter by use of asubnozzle mounted to the main nozzle and electrically insulatedtherefrom; disabling means for disabling the short-circuit detectingcircuit to render the predetermined detection output therefromineffective after the start of the wire extension until the wireelectrode reaches a predetermined delivery position; and discriminationmeans for determining that the wire extension ended in failure when thepredetermined detection output is generated by the short-circuitdetecting circuit after the short-circuit detecting circuit is releasedfrom a disabled state.
 2. An apparatus for detecting a failure inautomatic wire extension according to claim 1, including:a voltagesource connected to an input side of the short-circuit detectingcircuit.
 3. An apparatus for detecting a failure in automatic wireextension according to claim 2, including:a first switch for connectingthe voltage source and the input side of the short-circuit detectingcircuit to one of the main nozzle and the wire electrode; and a secondswitch for selectively causing the wire electrode to be grounded;wherein said wire cut electric discharge machine is operable to carryout a second wire extension through a machining initial hole having anormal diameter by using the main nozzle; and said discrimination meansbeing arranged to determine whether the first-mentioned wire extensionor said second wire extension is being executed, and control operatedpositions of the first and second switches in dependence on results ofthe determination.
 4. An apparatus for detecting a failure in automaticwire extension according to claim 1, wherein said wire cut electricdischarge machine is operable to carry out a second wire extensionthrough a machining initial hole having a normal diameter by using themain nozzle;said discrimination means being arranged to determinewhether the first-mentioned wire extension or said second wire extensionis being executed; said wire extension failure detecting apparatusfurther including a second short-circuit detecting circuit forgenerating a second predetermined detection output when a short circuitoccurs between the wire electrode and a workpiece during execution ofthe second wire extension; and said discrimination means being arrangedto determine that the wire extension ended in failure when the secondpredetermined detection output is generated.
 5. An apparatus fordetecting a failure in automatic wire extension according to claim 4,including:a voltage source connected to an input side of thefirst-mentioned short-circuit detecting circuit and an input side ofsaid second short-circuit detecting circuit.
 6. An apparatus fordetecting a failure in automatic wire extension according to claim 5,including:a first switch interposed between the voltage source and theinput side of the first-mentioned short-circuit detecting circuit; asecond switch having one end connected to a connection node between thevoltage source and the input side of the second short-circuit detectingcircuit, and having another end grounded; and a logic circuit interposedbetween the discrimination means and the first and second short-circuitdetecting means; and said discrimination means being arranged to controloperated positions of the first and second switches and operation of thelogic circuit, depending on whether the first-mentioned wire extensionor the second wire extension is being executed.